The present invention relates to tachometers for use in tape drives and the like and pertains particularly to an improved tachometer and method of manufacturing the same.
Tape drives are widely used as data storage devices for computers. It is essential when using tape drives for storage that the blocks of data be readily accessible. It is therefore important to determine precisely how much tape has been fed from one reel to another. The speed of the tape and its tension must also be precisely controlled.
Various tachometers have been used to determine the position of the tape, and to control its speed and tension. One widely used tachometer and drive assembly 10 is illustrated in FIG. 1 of the drawings. This assembly comprises a drive motor 12 having a drive shaft 14 on which a tachometer disc 16 is mounted. The tachometer disc is mounted on the shaft of the drive motor and passes between spaced emitter-detector pairs, which are connected in a quadrature arrangement in a sensing unit 18 to detect the direction and speed of the motor shaft. The disc 16 has a plurality of radially extending markings or slots 20 that are sensed by the emitter-detector pairs in the sensing unit 18. The disc is mounted between a first hub 22, which is mounted on the shaft 14, and a second disc or hub 24. The disc is constructed from thin copper shim stock (2 mil.) with the plurality of radial marking slots formed by etching. A center hole is then formed in the disc for mounting on the hub.
In accordance with the conventional construction, encoder disc 16 is manually mounted on the first hub 22, and is centered by the assembler observing it under a microscope while rotating and tapping the edge of the disc to precisely center it. Once the disc is centered, it is fixed in place between the hubs 22 and 24 by means of a drop of glue or other adhesive.
This process of assembly not only takes time but results in a fairly high percentage of rejects. Presently, assembly by this process takes from twenty to forty minutes, and the reject rate is frequently as high as forty percent.
The centering of the encoder disc is highly critical because inaccuracy in the centering can result in imprecise resolution of the markings. For example, with reference to FIG. 2, the spaced apart photo cells 1 and 2 are positioned so that a pair of spaced markings are out of phase precisely ninety degrees. This enables the sensing unit 18 to establish the direction and speed of rotation of the disc. Should the disc be out of radial or concentric alignment by as much as a few thousandths of an inch, the markings on the encoder disc will not be properly phased and the sensing unit generates inaccurate position data.
In addition, the encoder disc must be accurately centered vertically between the emitter-detector pairs. For example, the running gap between the disc and one side of each pair must be on the order of 0.001 to 0.005. Should the gap become greater than this, then the shadows of the encoded lines generate confusing signals. Also, should the gap become to small and rubbing occur between the disc and the sensing unit, the slot markings may become filled with debris, resulting in a failure of a sensed signal.
The present invention was devised to overcome the above problems of the prior art.